1. Field of the Invention
This invention relates to a liquid crystal display device, and in particular to a liquid crystal display device that is capable of preventing the deterioration of image quality due to the on-current decrease of a thin film transistor.
2. Description of the Related Art
Recently, a liquid crystal display device has been extensively utilized as a flat type display device. In particular, an active matrix type liquid crystal display device has attracted attention among various type of liquid crystal display devices because this active matrix type liquid crystal display device is exceptionally excellent in contrast ratio and also in response speed as compared with other types of liquid display device.
FIG. 1 shows a plan view of a typical example of the conventional active matrix type liquid crystal display device, which will be explained as follows.
Referring to FIG. 1, an array substrate 10 comprises a glass substrate 11 constituting an insulating transparent substrate. On this glass substrate 11 is formed a predetermined pattern of a conductive film made of molybdenum-tantalum (MoTa) alloy. This conductive film pattern is composed of an address line 12 including a gate electrode 12a, and an storage capacity electrode 13. On this glass substrate 11 comprising the address line 12, including the gate electrode 12a, and the storage capacity electrode 13 is formed a first gate insulating film consisting of silicon oxide (SiOx) formed by means of the atmosphere pressure CVD (Chemical Vapor Deposition) method.
On a portion of this first gate insulating film, which corresponds to where the gate electrode 12a is located, the following are superimposed by means of a plasma CVD method: a laminated pattern of a second gate insulating film consisting of silicon nitride (SiNx), a first semiconductor layer consisting of an amorphous silicon (a-Si) film and an etching stopper layer 17 functioning as a protecting film. Further, over the first semiconductor layer and the etching stopper 17 is formed a second semiconductor layer 18 consisting of a low resistance amorphous silicon (n.sup.+ a-Si).
Further, on another portion of the first gate insulating film 14, which corresponds to where the gate electrode 12a is not located, a pattern of display pixel electrode 19 is superimposed, by means of a sputtering method.
On the second semiconductor layer 18 is formed, through the sputtering of aluminum, patterns of signal line 20 functioning also as a source electrode 20a and of a drain electrode 21, one end of which is connected to the display pixel electrode 19, thereby constituting a thin film transistor.
Over this thin film transistor is formed a pattern of a passivation or protecting film 23 covering the thin film transistor as well as the portions around the display pixel electrode 19 with a space A being interposed between the circumference of the display pixel electrode 19 and the brim of the pattern of the passivation film 23 surrounding the display pixel electrode 19. Further, an alignment film is formed on passivation film 23.
Meanwhile, a counter substrate (not shown) is disposed facing the array substrate 10 which is explained above. This counter substrate comprises a glass substrate constituting an insulating transparent substrate. On a surface portion of this glass substrate is formed a pattern of color filters comprising red (R), green (G) and blue (B), each being formed on a portion corresponding to where the display pixel electrode 19 is formed. On the spaces between the neighboring color filters is formed a lattice-like pattern of black matrix so as to be disposed over the thin transistor. Further, on these color filters and black matrix is formed a counter electrode consisting of ITO on which an alignment film is formed.
On the outer surfaces of the glass substrate 11 of the array substrate 10 and of the glass substrate 31 of the counter substrate, which are opposite to the inner surfaces thereof and face each other, are formed polarizing plates respectively.
Between the array substrate 10 and the counter substrate is disposed a spacer which keeps the array substrate 10 spaced apart by a predetermined distance from the counter substrate. The peripheral portion of the space between the array substrate 10 and the counter substrate is sealed with a sealing agent 39 comprising, for example, epoxy resin. An opening is formed as shown in FIG. 2 at a portion of this sealing agent 39 thereby constituting an inlet port 40 for a liquid crystal. A liquid crystal 41 is filled through this inlet port 40 in the space between the array substrate 10 and the counter substrate. This inlet port 40 is also closed with a closing agent 42 comprising for example acrylic resin.
In this array substrate 10 of the conventional type, a space is formed between the display pixel electrode 19 and the protecting film 23 as shown in FIG. 1 so as to prevent the protecting film 23 from being overlapped by the display pixel electrode 19 in order to keep constant the voltage to be applied to the liquid crystal over the display pixel electrode 19 and to enlarge the aperture ratio of the display pixel electrode 19 as much as possible. Specifically, a groove-like space A is formed between the display pixel electrode 19 and the protecting film 23 in a manner to entirely surround the display pixel electrode 19, thus exposing through this space A a first gate insulating film 14.
This first gate insulating film 14, however, is vulnerable to the penetration of impurity ions and poor in moisture resistance so that impurity ions such as sodium (Na) generated from the glass substrate 11, for example, may be penetrated from this exposed portion A of the first gate insulating film 14 into the first gate insulating film 14. Similarly, water may enter through the liquid crystal inlet port 40, which is closed with the closing agent 42, or through the sealant 39 and may penetrate into the first gate insulating film 14. These impurity ions and water that has penetrated into the first gate insulating film 14, or even water originally included in the first gate insulating film 14, are more likely to be transferred into the thin film transistor, thereby inviting the deterioration in performance of the thin film transistor as well as the deterioration with time of the write performance due to the poor transistor, and thus giving rise to the problem of deteriorating image quality.
This deterioration of image quality--for example, a non-black portion is developed even if a signal to display a black image is given to the entire image screen in the normally white mode--occurs, in the initial stage, at the region within a radius of about 3 cm around the liquid crystal inlet port 40 as indicated by W in FIG. 3. This deteriorated portion W expands with time to the periphery of the sealing agent 42 as indicated in FIG. 4, thereby causing the deterioration of image visibility.
Therefore, there has been a strong demand to develop a liquid crystal display device which is free from the problems as explained above and capable of improving the reliability, and can be manufactured without necessitating an increase of patterning steps and with an excellent alignment.